The invention concerns a method for wavelength calibration of an electromagnetic radiation filtering device included in an apparatus measuring spectral transmission of a propagating medium external to said apparatus and wherein said radiation flows through.
Known"" apparatuses such as gas analyzers, apparatuses for measurement of the calorific value of a gas or for example gas sensors comprises:
at least one source of radiation,
a filtering device which shows spectral transmission tunable on a wavelength range of said radiation based on the value of a physical parameter,
a device for detection of radiation emitted by the source, the radiation source and said detection device being separated by said propagating medium.
One characteristic of a tunable filtering device is the relation between the value V of the physical parameter applied to said device and the central wavelength xcexmax corresponding to the transmittance maximum of the filtering device.
The relation xcexmax (V) can, for example, be determined using a Fourier transformation spectrometer by measuring the transmittance of the filtering device at different values V of the physical parameter applied to said device and then identifying the corresponding central wavelength values for which transmittance of the filtering device is at a maximum.
FIG. 1 represents the wavelength spectral transmission T of a filtering device tuned on several central wavelengths obtained for values V1, V2 of the physical parameter.
The wavelength calibration process is generally carried out in the laboratory and calibration therefore depends on the internal features of the spectrometer.
The apparatus is then installed on-site.
It has been noted that, during use of the apparatus, and therefore of the filtering device, the relation xcexmax (V) between the central wavelength for which transmittance of the filtering device is at a maximum and the control value V of the filtering device can change.
Such a change can be explained, for example, by the fact that during its use, the filtering device is subject to a temperature that is different from the temperature conditions present during calibration.
Such a change can also result from aging of the material(s) which the filtering device is made from.
Should this be the case, all that can be done is to remove the apparatus from its location and carry out another calibration of the filtering device in the laboratory, as described above, then to re-install the apparatus on-site with the re-calibrated filtering device.
It would therefore be beneficial to find a wavelength calibration method that would resolve at least one of the following two problems: carrying out calibration in the laboratory without having to use a Fourier transformation spectrometer or carrying out the calibration without having to remove the apparatus from its location.
To this end, this invention proposes a wavelength calibration method for an electromagnetic radiation filtering device included in an apparatus measuring the spectral transmission of a propagation medium external to said apparatus and wherein said radiation flows through, said filtering device having spectral transmission tunable on a wavelength range of said radiation based on the value of a physical parameter, said method comprises the steps of:
selecting at least one absorbing gaseous line which is always present in natural form in the propagation medium and whose corresponding wavelength is included in said filtering device tunability wavelength range,
and in calibrating the filtering device with respect to said at least one absorbing gaseous line which is used as a natural reference.
This method is particularly easy to apply since it does not require modification of the apparatus in which the filtering device is included, for example by including a cell containing a reference gas.
Preferentially, said at least one absorbing gaseous line has a spectral width less than or equal to that of the filtering device and is sufficiently intense not to be masked by other gaseous lines.
Such a method can therefore be advantageously used to calibrate a filtering device when the apparatus in which the device is included is installed at the site where it is used.
With this method, it is no longer necessary to transport the apparatus to a laboratory to carry out its calibration since the absorbing gaseous line used as a reference is naturally present in the propagating medium.
The propagating medium can be, for example, the atmosphere and the apparatus, a carbon monoxide sensor using the carbon dioxide rays in the atmosphere as a natural reference(s).
Preferably, the apparatus also comprises:
at least one source of electromagnetic radiation and
a device for detection of radiation emitted by the source, said source and said detection device being separated by the propagating medium.
It is also possible to interpose the volume of gas whose spectral transmission is to be measured between the source and detection device and for this volume of gas to contain absorbing gaseous lines that can be used as natural references in accordance with the method of the invention.
In this case, the interposed volume of gas acts as the propagating medium in the sense of the invention.
If the interposed volume of gas does not occupy the entire volume between the source and detection device, it is then also possible to choose from natural gaseous lines in the gas volume and natural gaseous lines in the remaining unoccupied volume between said source and said device, those lines that are to be used.
This method can also be applied in the laboratory to calibrate the filtering device before it is used for the first time without having to use a Fourier transformation spectrometer.
More particularly, the method according to the invention consists in successively:
varying the physical parameter applied to the filtering device so that the spectral transmission maximum of said filtering device coincides with the wavelength of the reference gaseous line,
deducing the coefficient(s) of the law governing the tunability of the wavelength filtering device, the general behavior of said law being known in advance,
determining from this law other values of the physical parameter each corresponding to a wavelength range on which the spectral transmission of the filtering device is tuned during use.
It is possible for example to choose from said range of electromagnetic radiation wavelengths the absorbing gaseous line which has the greatest intensity with respect to the other absorbing gaseous lines.
During calibration of the filtering device, identification of this gaseous line is straightforward because it corresponds to maximum absorption in the wavelength range.
It is also advantageous to choose two absorbing gaseous lines from said range of electromagnetic radiation wavelengths rather than a single one for greater reliability of calibration.
Preferentially, electromagnetic radiation is of the infrared type.
One of the absorbing gaseous lines is, for example, that of methane at 1.666 microns.
It can also be useful to choose the absorbing line of methane at 1.791 microns, depending on the envisaged wavelength range and tunability range of the filtering device.
Preferentially, the method consists in applying an electrical field in the form of electrical voltage to the filtering device as a physical parameter but a magnetic field can also be used.
According to other characteristics:
the filtering device is a Fabry-Perot interferometer,
the Fabry-Perot is a short interferometer,
the Fabry-Perot interferometer is a micro-machined interferometer,
the apparatus is a gas analyzer,
the apparatus is an apparatus for measurement of the calorific value of a gas,
the apparatus is a gas sensor.